As is well known, the manufacturing process of various kinds of electronic or semiconductor devices such as ICs, LSIs, and the like, involves a fine patterning of a resist layer on the surface of a substrate such as a semiconductor silicon wafer. This fine patterning process has traditionally been conducted by the photolithographic method in which the substrate surface is uniformly coated with a thin layer of a positive or negative tone photoresist composition. The photoresist composition is then selectively irradiated with actinic radiation (such as ultraviolet light) through a photomask (which may be a passive mask such as a chrome pattern on glass, an active mask such as mirrors to selectively deflect the radiation, or a virtual mask formed by selectively deflecting charged particle radiation using for instance electromagnets or electrostatic fields). The irradiation is followed by a development treatment which selectively dissolves away the photoresist layer in the areas exposed (in the case of positive tone photoresists) or unexposed (in the case of negative tone photoresists) to the actinic radiation, to leave a patterned resist layer on the substrate surface. The patterned resist layer is then utilised as a mask in subsequent treatment (such as etching) of the substrate surface.
The fabrication of structures with dimensions of the order of nanometres is an area of considerable interest, since it enables the realisation of electronic and optical devices which exploit phenomena such as quantum confinement effects, and also allows greater component packing density. As a result, the resist layer is required to have an ever-increasing fineness which can be accomplished only by using actinic radiation having a shorter wavelength than the conventional ultraviolet light. Accordingly, it is now the case that electron beams (e-beams), excimer laser beams, EUV and X-rays are commonly used as the short-wavelength actinic rays. The minimum size of structures obtainable is determined both by the wavelength of the actinic radiation, and by the performance of the resist material used.
Various materials have been proposed as suitable resist materials. These include organic resinous materials such as methacrylic resin-based, polystyrene-based, and novolac resin-based materials. In the case of negative tone resists based on polymer crosslinking, there is an inherent resolution limit of about 10 nm, which is the approximate radius of a single polymer molecule.
It is also known to apply a technique called ‘chemical amplification’ to the resist materials. A chemically amplified resist material is generally a multi-component formulation, in which there is a main resist component, such as novolac resin which contributes towards properties such as resistance of the material to etching and mechanical stability; optionally one or more additional components which impart desired properties to the resist; and a sensitizer. By definition, the chemical amplification occurs through a catalytic process involving the sensitizer, which results in a single irradiation event resulting in the ‘exposure’ of multiple molecules of the main resist component. In a typical example, the resist material comprises a polymer (the main resist component) and a photoacid generator (PAG) (the sensitizer). Upon irradiation (with actinic radiation), the PAG releases a proton which then reacts with a molecule of the polymer to cause it to lose a dissolution-inhibiting group (in the case of a positive tone resist). In the process, a second proton is generated which can then react with a further molecule. The speed of the reaction can be controlled, for example, by heating the resist film to drive the reaction. The reacted polymer molecules are soluble in a ‘developer’ (solvent), and hence can be removed whilst the unreacted polymer is not, and remains in place as a patterned resist layer. In this way the sensitivity of the material to actinic radiation is greatly increased, as small numbers of irradiation events give rise to a large number of exposure events.
Published International Patent Application WO 2006/030234 A2 describes the use of certain methanofullerene derivatives as negative-tone photoresists in compositions with a photoacid generator and an epoxy novolak crosslinker. Such photoresists showed good sensitivity, resolution and high etch durability. However, development of the irradiated resist requires the use of toxic organic solvents, which is disadvantageous in an industrial context.